Field of the Invention
The present invention relates to golf balls. Specifically, the present invention relates to improvement of aerodynamic characteristics of golf balls.
Description of the Related Art
Golf balls have a large number of dimples on the surfaces thereof. The dimples disturb the air flow around the golf ball during flight to cause turbulent flow separation. This phenomenon is referred to as “turbulization”. Due to the turbulization, separation points of the air from the golf ball shift backwards leading to a reduction of drag. The turbulization promotes the displacement between the separation point on the upper side and the separation point on the lower side of the golf ball, which results from the backspin, thereby enhancing the lift force that acts upon the golf ball. Excellent dimples efficiently disturb the air flow. The excellent dimples produce a long flight distance.
A polyhedron is used for arranging dimples. The polyhedron is inscribed in the phantom sphere of a golf ball. A large number of sides of the polyhedron are projected onto the surface of the phantom sphere by light that travels from the center of the phantom sphere in the radial direction. By this projection, a large number of comparting lines are obtained on the surface of the phantom sphere. By these comparting lines, the surface of the phantom sphere is divided into a large number of units (spherical polygons). A large number of dimples are arranged on one unit to obtain a dimple pattern. The dimple pattern is developed to the other units to obtain a dimple pattern of the entire golf ball. The dimple pattern is referred to as polyhedron pattern. The polyhedron pattern has a large number of symmetric axes. A golf ball having a polyhedron pattern is disclosed in JPH1-221182 (U.S. Pat. No. 5,078,402).
A dimple pattern referred to as hemisphere division pattern is used in commercial golf balls. In designing of this pattern, first, a hemisphere (a half of a phantom sphere) is divided into a plurality of units by a plurality of longitude lines. The shape of each unit is a spherical isosceles triangle. A large number of dimples are arranged on one unit to obtain a dimple pattern. The dimple pattern is developed to the other units. The development is achieved by rotating one unit pattern about a line passing through the north pole and the south pole. By this rotation, a dimple pattern of the entire golf ball is obtained. The pattern of the golf ball is rotationally symmetrical.
JP2013-9906 (US2013/0005510) discloses a golf ball having dimples which are randomly arranged. The contour shape of each dimple is non-circular. In the golf ball, the ratio of the total area of the dimples relative to the surface area of the phantom sphere of the golf ball is high. This ratio is referred to as occupation ratio. The flight distance performance of the golf ball correlates with the occupation ratio. The golf ball has excellent flight distance performance.
In the polyhedron pattern, the arrangement of the dimples is constrained by the comparting lines. In the polyhedron pattern, the dimples are less likely to be densely arranged. The flight distance performance of a golf ball having a polyhedron pattern is not sufficient.
In the hemisphere division pattern, the distribution of the dimples is ununiform. Therefore, the aerodynamic symmetry of a golf ball having a hemisphere division pattern is not sufficient.
The golf ball disclosed in JP2013-9906 also has inferior aerodynamic symmetry.
An object of the present invention is to provide a golf ball having excellent flight distance performance and aerodynamic symmetry.